Manufacture of iron powder



p 1958 v L. SCHLECHT ETAL 2,851,347-

MANUFACTURE OF IRON POWDER Filed March 2, 1956 WITHDRAWAL OF 60 CARBONYLVQPOR WITHDRYAVIAL OF METAL mm ens mvzmbas LEO SCH LECHT ERNSTOESTREICHER FRIEDRICH BERGMANN BY W W United States Patent 2,851,347MANUFACTURE or IRON rownnn Leo Schlecht, Ludwigshafen (Rhine), ErnstOestreicher, Lndwigshafen (Rhine) Oppau, and Friedrich Bergmann,

Ludwigshafen (Rhine), Germany, assignors to Badische Anilin- &Soda-Fabrik Aktiengesellschaft, Ludwigshafen (Rhine), GermanyApplication March 2, 1956, Serial No. 569,210

Claims priority, application Germany October 21, 1949 4 Claims. (Cl.75-.5)

Iron powder having a very small particle size is espev cially desirablefor use in magnetic materials in the electrical field. Besides asuitable carbon content, the particle size is very important for highfrequency and ultra high frequency applications, and for these purposes,iron powder having a particle size which does not exceed 5 microns ishighly desirable.

Such, very fine iron powder alsoaifords considerable advantages inpowder metallurgical applications. The fine powders sinter at lowtemperatures, and sintered articles are produced which, unlike sinteredarticlesmade from coarser powders, can be transformed without difficultyby pressure methods, such as hammering or rolling, intosemifinishedproductswithout the formation of undesirable cracks. The fine powdersmarkedly facilitate the formation of alloys with other metal powders,enabling the rapid production of homogeneous alloys and, where desired,a high pore volume.

The principal object of-this invention is to provide an improved methodor process for producing iron powder in which the particle size of atleast about 80 percent by weight of the particles does not exceed 5microns and,

particularly, to avoid the special measures which were previouslyrequired and provide a very simple process which markedly increases thequantity of fine powder produced.

The invention is particularly concerned with the production of ironpowder by thermal decomposition of iron carbonyl vapor in the hotfreespace of a decomposition chamber or vessel. 'In accordance with theinvention, the surprising discovery has been made that iron powder inwhich the particle size of at least about 80 percent by weight of theparticles does not exceed 5 microns is produced by decomposing 'at leastabout 22 kilograms per hour of iron carbonyl per cubic meter ofdecomposition space. The decomposition temperature is maintained withinthe range of 200 C. to 300 C.

In the same manner there is produced iron powder in which the particlesize of substantially all of the particles does not exceed 5 microns bydecomposing at least about 30 kilograms per hour of iron carbonyl percubic meter of decomposition space. In this manner, particles below 5microns. are produced which have a low carbon contentand good magneticproperties, especially for high frequency purposes; at the same time,the quantity produced is much greater than by prior methods.

Iron powder characterized by a very small particle size, in whichsubstantially all of the particles do not exceed 3 microns, is producedby decomposing at least about 43 kilograms of iron carbonyl/hr./m. Theparticle size is further reduced so that substantially all of PatentedSept. 9, 1958 ICC the'particles do not exceed 2 microns by decomposingat least about 46 kg./hr./m Substantially all as em ployed herein meansabout or greater.

At the same time, the temperature in the decomposition space ismaintained within the range of 200 C. to 300 C., to produce the desiredlow carbon content and good magnetic properties. Preferably, thedecomposition temperature is maintained within the range of 230 C. to280 C., and it is further preferred to carry out the decomposition atabout 260 C. to 270 C. The carbonyl vapor is also preferably mixed witha small amount of ammonia, in known manner, to reduce the quantity offree carbon in the powder.

Part of the heat required may be supplied directly in the decompositionspace, and at least part is supplied through the wall of thedecomposition chamber. The heat supplied through the wall of the chamberis supplied in such a manner that the temperature of the inner surfaceof the wall is above the decomposition temperature of the carbonyl butnot substantially higher than thedecomposition temperatureof carbonmonoxide.

Prior to the invention, a number of processes have been proposed forproducing finely divided iron from iron carbonyl vapor, wherein vaporousor gaseous iron carbonyl was introduced into a chamber externally heatedto temperatures above the decomposition temperature of the carbonyLthedecomposition taking place substantially in the hot free space of thechamber at a distance from the hot Walls of the chamber. In this manner,iron powder was produced having a particle size predominantly between 2and 10 microns, with a considerable proportion larger than 5 microns.

T o produce the powder below 5 microns in particle size desired for highfrequency purposes, several methods have been proposed. For example, onemethod involved the dilution of the iron carbonyl vapor with thirty ormore volumes of carbon monoxide. In this manner, there was an effect onsize due to dilution and the gases passed through the decompositionchamber at 30 or more times the speed of the gases when iron carbonylalone was supplied to the chamber. Another method involved applyingreduced pressure from the chamber exit, for example, down to 20millimeters of mercury, which correspondingly increased the speed ofpassage of the materials through the chamber. Also, it was proposed tolimit the time of the iron particles in the decomposition zone to 20seconds. These measures, while producing fine particles, are limited bythe low quantitative production of iron powder. They also requirespecial measures for carrying out the decomposition, for recovering theiron powder, and for purifying the iron powder, especially whenundecomposed carbonyl remains.

Prior to the present invention, it was thought'that the quantitativedecomposition of iron carbonyl per unit of time and space was limited,due to the previously observed increase in carbon content, which madethe powder unsuitable for high frequency applications.

The process of the invention may be carried out in an apparatus, such asthat illustrated in the accompanying drawing. There, a decompositionchamber 4 is illustrated which is heated by a hollow jacket 2surrounding the chamber through which heating gases circulate. The heatnecessary for the decomposition of iron carbonyl is produced in the oilor has burner 5 into which oil or gases to'be burnt are introducedthrough the pipe 7. The hot gases then are introduced into the jacket 2through the pipe 8 and are led around the chamber by means of baffleplates 3 and drawn off at the lower part of the jacket by means of ablower 6 from which the gases are recycled into the burner 5 in order tobe heated again. The iron carbonyl vapor is introduced into the chamberthrough pipe 1 and is decomposed into iron powder and carbon andwithdrawn through a valve 10. From the chamber the iron powder is drawnoff through the valve 14. In the present invention, it is necessary toinsure that sulficient heat be supplied to the decomposition space inthe chamber to substantially completely decompose the quantity of atleast 22 kilograms of iron carbonyl per hour per cubic meter supplied tothe chamber. In providing this large quantity of heat to the interior,to efiect the much greater quantity of decomposition, it is importantthat the inner wall of the decomposition chamber be at a temperature nothigher than that which will produce carbon by decomposition of carbonmonoxide. In other words, the temperature differential, or At, from theinner wall to the interior of the decomposition space is not raised, butthe quantity of heat transmitted to the space is increased. This isaccomplished, for example, by sup- .plying a greater quantity of heatinggases to the jacket,

but at no higher temperature. Alternatively, the heat transmittingsurface of the chamber wall may be enlarged, or a more conductivematerial may be employed in the construction of the wall. On the otherhand, increasing the quantitative heat transferred by increasing thetemperature of the heating medium results in increased inner walltemperature and the production of iron powder having an unsuitablecarbon content and magnetic properties.

The process of the present invention can be used together withappropriate prior methods for producing fine particles, to furtherreduce the particle size, if desired. Thus, the iron carbonyl vapor canbe diluted with a vapor or gas, such as carbon monoxide. Even a 1:1dilution has a favorable efiect. In this manner, by decomposing at least22 kilograms per hour of iron carbonyl per cubic meter of space,together with dilution of the carbonyl vapor, it is possible to obtain apowder having an exceptionally small apparent density, which forms fiocsconsisting of particles usually less than 1 micron in size. Also, whendiluting the carbonyl vapor with carbon monoxide, the carbon monoxidecan be heated and thus supply part of the heat necessary for thedecomposition and reduce the requirements for heat transmitted throughthe wall of the decomposition chamber.

While observing the above described conditions, the decomposition may becarried out by applying a reduced pressure through the exit from thechamber or causing the decomposition to take place againstsuperatmospheric back pressure. However, an important advantage of theinvention is that the decomposition may be carried out at aboutatmospheric pressure, that is, at the pressure existing in the-systemwhen the gases exit to atmospheric pressure with no application ofreduced or increased pressure.

While the residence time of the particles in the decomposition zone maybe reduced according to prior methods, such a condition is unnecessary,as the iron particles may remain in the decomposition zone for, e. g.,100 seconds without adverse effect.

The following examples illustrate the invention and also illustrate thesurprising and unexpected nature thereof, but it is to be understoodthat the invention is not limited to the specific conditions andprocedures set forth therein, which are only illustrative.

Example 1 A decomposition chamber, as shown in the figure, was providedwhich had an inner diameter of 1 meter and a length of 5 meters, andwhich was surrounded with a jacket for heating the chamber by thecirculation of hot gases in the jacket. The hot gases come from asuitable furnace and were blended or mixed with a portion of the coolerspent heating gases leaving the jacket, to maintasm the temperature inthe decomposition space at about 2 0 C.

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60 kilograms of iron carbonyl vapor diluted with ammonia in an amountcorresponding to 4% by volume of the carbon monoxide originating fromthe decomposition of the carbonyl were supplied to the decompositionchamber per hour. This was equivalent to a throughput of about 15kilograms per hour per cubic meter or decomposition space, which waspreviously believed to be the maximum attainable to produce an ironpowder containing particles which are suitable for high frequency coresand the like. The gases exited to substantially atmospheric pressure. Aniron powder having the following particle size distribution wasobtained:

of iron carbonyl containing the same relative proportion of ammonia weresupplied to the decomposition chamber per hour. This corresponded to asupply of about 22 kilograms per hour per cubic meter of decompositionspace. The gases left the chamber to substantially atmospheric pressure.The heating gas supplied to the jacket was at the same temperature asbefore but the quantity of heating gas supplied was about 50% greater.The temperature of the decomposition space was about the same as inExample 1.

A much finer powder having the following particle size distribution wasobtained:

. Microns 82% up to 5 18% 5-6 The average carbon content of theparticles was 0.71%.

Example 3 I Microns up to 3 25 3-5 The average carbon content of theparticles was 0.69%.

Example 4 Microns 37.4% up to 1 29.6% 1-2 20.4% 2-3 8.3% 3-4 The heatinggas supplied to the v Example 5 Observing the above conditions, as inExample 4. 175 kilograms of iron carbonyl were decomposed per hour.

The iron powder obtained had the following particle size distribution:

' Microns 76.5% up to 1 13.2% 1-2 7.8% 2-3 The carbon content andmagnetic properties were likewise as desired for high frequencyapplications.

Example 6 In like manner, the amount of iron carbonyl decomposition wasincreased to 184 kilograms per hour. The iron powder particle sizedistribution was as follows:

Microns 77% up to 1 18% l-2 4% 2-3 The carbon content and magneticproperties were likewise suited for high frequency purposes.

It will be observed on comparison of this example with Examples .4 and 5that the proportion of smaller particles increases with increasingquantity of iron carbonyl decomposition. In particular, over threefourths of the particles have a size of 1 micron or less andsubstantially 90% or more of the particles have a size of 2 m1- crons orless at a decomposition rate of 175 kilograms per hour or greater,corresponding to a quantitative decomposition of about 45 kilograms perhour per cubic meter. Further reduction in particle size can be obtainedby increasing the quantity of iron carbonyl decomposition per unit oftime and volume. These extremely line iron powders are especiallysuitable for the preparation of cores for the radio and televisionindustries.

The invention thus embodies the discovery of a new phenomenon andcondition in the production of fine iron powder, that the iron carbonylbe decomposed in a hitherto unknown quantity per unit of time andvolume. The invention provides a great increase in the production ofthe-very fine iron powders, by an exceptionally simple and very'reliable method. The powders can be used without further treatment forthe manufacture of high which comprises supplying at least about 30kilograms per hour of iron carbonyl per cubic meter of decompositionspace to said free space, and substantially completely decomposing saidquantity of iron carbonyl therein, by maintaining the decompositiontemperature within the range of 200 C. to 300 C. and supplying sufficienheat ,to said free space to produce said complete decomposition withinsaid temperature range.

2. In the production of iron powder by thermal decomposition of ironcarbonyl vapor in the hot free spate: of a decomposition chamber, theimprovement for producing iron powder in which the particle size ofsubstantially all of the particles does not exceed 5 microns whichcomprises supplying at least about '30 kilograms per hour of ironcarbonyl per cubic meter of decomposition space to said free space atabout atmospheric pressure, and substantially completely decomposingsaid quantity of iron carbonyl therein, by maintaining the decompositiontemperature within the range of 200 C. to 300 C. and supplyingsufficient heat to said free space to produce said completedecompositionwithin said tempcrature range.

3. In the product-ion of iron powder by thermal decomposition of ironcarbonyl vapor in the hot free space of a decomposition chamber, theimprovement for producing iron powder in which the particle size ofsubstantially all of the particles does not exceed 5 microns whichcomprises supplying at least about 30 kilograms per hour of ironcarbonyl per cubic meter of decomposition space to said free space, andsubstantially completely decomposing said quantity of iron carbonyltherein, by main.- taining the decomposition temperature within therange of 230 C. to 280 C. and supplying suflicient heat to said freespace to produce said complete decomposition within said temperaturerange.

4. In the production of iron powder by thermal decomposition of ironcarbonyl vapor in the hot free space of a decomposition chamber, theimprovement for producing iron powder in which the particle size ofsubstantially all of the particles does not exceed 3 microns whichcomprises supplying at least about 43 kilograms per hour of ironcarbonyl per cubic meter of decomposition space to said free space, andsubstantially completely decomposing said quantity of iron carbonyltherein, by maintaining the decomposition temperature within the rangeof 200 C. to 300 C. and supplying sufficient heat to said free space toproduce said complete decomposition within said temperature range, atleast part of the heat required being supplied through the wall of saidchamber, the temperature of the inner surface of said wall beingmaintained not substantially higher than the decomposition temperatureof carbon monoxide.

References Cited in the tile of this patent UNITED STATES PATENTSSchlecht et al Dec. 15, 1931 Beller May 20, 1952 OTHER REFERENCES

1. IN THE PRODUCTION OF IRON POWDER BY THERMAL DECOMPOSITION OF IRONCARBONYL VAPOR IN THE HOT FREE SPACE OF A DECOMPOSITION CHAMBER, THEIMPROVEMENT FOR PRODUCING IRON POWDER IN WHICH THE PARTICLE SIZE OFSUBSTANTIALLY ALL OF THE PARTICLES DOES NOT EXCEED 5 MICRONS WHICHCOMPRISES SUPPLYING AT LEAST ABOUT 30 KILOGRAMS PER HOUR OF IRONCARBONYL PER CUBIC METER OF DECOMPO-